This invention relates to leaky coaxial cables such as are used for guided communications, obstacle detection, and perimeter security. Specifically, the present invention relates to a leaky coaxial cable having a bonded outer shield formed by a conductive tape wound at a low pitch angle in a spiral path along the cable length.
Leaky coaxial cables, sometimes known as ported coaxial cables or radiating coaxial cables, are generally constructed with gaps or apertures in their outer shield which permit a portion of the internal field to couple to the external environment and external fields to couple to the cable. For example, U.S. Pat. No. 4,300,338 discloses a design with rhombic shaped apertures in the outer conductor. Both inductive and capacitive coupling is produced having a magnitude dependent on the size, shape, orientation and density of the apertures.
Leaky coaxial cables can be produced with thin, solid, tubular outer shields, as shown in U.S. Pat. No. 3,681,717, in which there is diffusion coupling through the shield due to its thickness being of the same order as, or smaller than, the skin depth at the frequency of operation. Finally, it is known that by use of a spiral or solenoidal construction path along the outer conductor inductive coupling can be produced with no aperture or gap necessarily being present. U.S. Pat. No. 3,735,293, for example, shows a cable having an outer conductor formed from closely wound metal tape with an insulating backing.
In the design of all such cables it is desired to produce a defined level of coupling with minimal effect on such coaxial cable parameters as impedance, velocity of propagation and downline attenuation. The primary components of attenuation in non-leaky cables are due to conductor and dielectric losses, but in leaky coaxial cables losses also occur due to coupling with the external environment. The presence of apertures, since they result from metal removal from the conduction path, cause an inherent increase in attenuation.
Models of coupled transmission lines indicate that the capacitive coupling inherent with apertures or longitudinal gaps is generally undesirable. This coupling varies with the dielectric constant of the materials external to the cable and, thus, produces undesirable environmental sensitivity. Capacitive may also reduce the signals transferred by inductive coupling by producing components of opposite phase to them. Finally, capacitive coupling also produces a loss which contributes to attenuation.
Diffusion coupling cables are limited in leaky cable applications both because the resulting coupling is weak and a substantial increase in attenuation results from the requirement that the thickness of the outer shield must be reduced.
Cables relying on a solenoidal conductive path in the outer conductor, called induction cables, have been restricted to use at low frequencies, because the resulting large inductive coupling increases linearly with frequency. This has been found to cause large mismatch effects and high coaxial attenuation due to a high degree of coupling when used in the frequency range of typical applications, greater than 30 MHz. Frequencies in the 30-200 MHz band are used for the detection of humans or obstacles which have a dimension of approximately 1/4 wavelength in this band. Also coaxial attenuation is inherently high for cables using high pitch angle conductors to produce the solenoidal currents since the conductor path is long. Typical application angles for spiral tapes in normal manufacturing practice is in the range of 30-70 degrees (e.g. U.S. Pat. Nos. 3,735,293, 3,949,329 and 3,870,977). Coaxial attenuation increases approximately as the inverse of the cosine squared of the pitch angle for full coverage spiral tapes.
For many applications it is desirable to be able to `grade` or modulate the cable coupling, as shown in U.S. Pat. No. 4,432,193, by varying some cable parameters with length. This can, for example, be used to compensate for cable attenuation so that the external field along the cable from the signal input is maintained of uniform magnitude.